[PATCH 1/6] mm: Allow per-VMA locks on file-backed VMAs

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

The fault path will immediately fail in handle_mm_fault(), so this
is the minimal step which allows the per-VMA lock to be taken on
file-backed VMAs.  There may be a small performance reduction as a
little unnecessary work will be done on each page fault.  See later
patches for the improvement.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/memory.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index fdaec7772fff..f726f85f0081 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5223,6 +5223,9 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 					    flags & FAULT_FLAG_REMOTE))
 		return VM_FAULT_SIGSEGV;
 
+	if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma))
+		return VM_FAULT_RETRY;
+
 	/*
 	 * Enable the memcg OOM handling for faults triggered in user
 	 * space.  Kernel faults are handled more gracefully.
@@ -5275,12 +5278,8 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
-	/* Only anonymous vmas are supported for now */
-	if (!vma_is_anonymous(vma))
-		goto inval;
-
 	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
-	if (!vma->anon_vma)
+	if (vma_is_anonymous(vma) && !vma->anon_vma)
 		goto inval;
 
 	if (!vma_start_read(vma))
-- 
2.39.2